Knowledge regarding the fundamental mechanisms underlying transport of bile salts and vitamin B-12 across the brush border and basolateral plasma membranes of ileal epithelial cells is of crucial importance in understanding intestinal absorption of these physiologically and nutritionally important substances. The goal of this investigation is to describe the molecular events, elucidate the energy sources and identify the membrane constituents which are involved in transfer of bile salts and vitamin B-12 from the lumen of the ileum to the interstitial fluid. For this purpose we will prepare membrane vesicles derived from intestinal epithelial cells and determine the characteristics of uptake of bile salts and vitamin B-12 by these vesicles. In addition, the characteristics of uptake of vitamin B-12 and intrinsic factor and their distribution among cellular organelles will be determined using isolated enterocytes. The effects of ions, ion gradients, adenosine triphosphate, temperature, competitive substrates and membrane potential upon vesicular and cellular binding and transport of bile salts and vitamin B-12 will be ascertained. We will also solubilize and purify the bile salt and vitamin B-12 transport system and incorporate them into phospholipid liposomes and planar artificial membranes. The reconstituted transport systems will be utilized to identify the membrane constituents which are responsible for transmembrane transfer of bile salts and vitamin B-12 and to determine how these transport systems interact with other components of the brush border membrane. The information obtained in this investigation will provide a fuller understanding of the functional principals governing intestinal absorption of bile salts and vitamin B-12 and should also contribute significantly to a broader understanding of transport physiology and pathophysiology in general.